Cardiopulmonary resuscitation (CPR) is a well-known and valuable method of first aid used to resuscitate people who have suffered from cardiac arrest. CPR requires repetitive chest compressions to squeeze the heart and the thoracic cavity to pump blood through the body. Artificial respiration, such as mouth-to-mouth breathing or a bag mask apparatus, is used to supply air to the lungs. When a first aid provider performs manual chest compression effectively, blood flow in the body is about 25% to 30% of normal blood flow. However, even experienced paramedics cannot maintain adequate chest compressions for more than a few minutes. Hightower, et al., Decay In Quality Of Chest Compressions Over Time, 26 Ann. Emerg. Med. 300 (September 1995). Thus, CPR is not often successful at sustaining or reviving the patient. Nevertheless, if chest compressions could be adequately maintained, then cardiac arrest victims could be sustained for extended periods of time. Occasional reports of extended CPR efforts (45 to 90 minutes) have been reported, with the victims eventually being saved by coronary bypass surgery. See Tovar, et al., Successful Myocardial Revascularization and Neurologic Recovery, 22 Texas Heart J. 271 (1995).
In efforts to provide better blood flow and increase the effectiveness of bystander resuscitation efforts, various mechanical devices have been proposed for performing CPR. In one variation of such devices, a belt is placed around the patient's chest and an automatic chest compression device tightens the belt to effect chest compressions. Our own patents, Mollenauer et al., Resuscitation device having a motor driven belt to constrict/compress the chest, U.S. Pat. No. 6,142,962 (Nov. 7, 2000); Bystrom et al., Resuscitation and alert system, U.S. Pat. No. 6,090,056 (Jul. 18, 2000); Sherman et al., Modular CPR assist device, U.S. Pat. No. 6,066,106 (May 23, 2000); and Sherman et al., Modular CPR assist device, U.S. Pat. No. 6,398,745 (Jun. 4, 2002); and our application Ser. No. 09/866,377 filed on May 25, 2001, our application Ser. No. 10/192,771, filed Jul. 10, 2002 and our application Ser. No. 12/726,262, filed Mar. 17, 2010 show chest compression devices that compress a patient's chest with a belt. Each of these patents or applications is hereby incorporated by reference in their entireties.
Since seconds count during an emergency, any CPR device should be easy to use and facilitate rapid deployment of the device on the patient. Our own devices are easy to deploy quickly and may significantly increase the patient's chances of survival.
One important aspect of such devices is the need for small, powerful yet reliable power supply to power the device. It is not uncommon for CPR to be administered for at least thirty minutes. Thus, the power supply must be capable of delivering sufficient energy to the motor driving the compression device for at least that length of time. Moreover, the power supply must be relatively light weight, so as to enhance portability of the chest compression device, yet it must deliver its power for an extended period of time without significant voltage or current drop off to ensure consistency of compression throughout the treatment period.
Various approaches to providing the type of high current power cell battery needed to power a mechanical compression device have been designed. As more efficient battery designs and chemistries have been used, the need to carefully manage the charging and discharging of the battery has arisen. To meet this need, complex battery management circuitry has been designed, including the use of processors, memory, and other components. All of these components need to fit within the confines of the battery casing designed for use in a piece of equipment to be powered.
To prevent inadvertent shorting of the battery terminals, one approach that has shown promise is to electrically isolate the battery cells from the terminals unless the battery is properly inserted into a device to be powered, a charger, or other authorized device. Such isolation requires the use of some type of switch which may be controlled by the battery management software and hardware.
One electronic switch that is suitable for such an application is a field effect transistor, or MOSFET. In typical designs, a p-FET device would be used as the switch and would be placed in the high side of a main bus to isolate the battery cells from the terminals. However, p-FETS have two or more times the on resistance of an n-FET device. Thus, to handle the same current as an n-FET device, several p-FETs wired in parallel would be needed. Moreover, use of several p-FETS may also require the use of a heat sink to dissipate heat generated while the p-FET is on. This is disadvantageous where space within the battery pack is limited.
N-channel FETs, or n-FETs, are a better choice for such a design because they are capable of handling the amount of current required. One problem, however, is that, the voltage required to drive the n-FETs closed exceeds the voltage that is available from the battery pack. For example, using an n-FET that requires a bias voltage of 10 volts, the n-FET requires a gate drive voltage of the bias voltage plus the battery voltage to drive the n-FET sufficiently to allow an appropriate voltage to pass through the n-FET to charge or discharge the battery.
What has been needed, and heretofore unavailable, is a reliable boost circuit for providing a sufficient gate drive voltage to a n-FET that is controlled by a processor that monitors the need for the n-FET, and responds to that need by enabling the boost circuit to control the conductive state of the n-FET. Such a circuit also needs to be fast and robust, and require the minimum number of components such as to fit within the confines of a battery case. The present invention satisfies these, and other needs.